Stimulated scattering of intense light is one of the major subjects in the nonlinear optics. Although several types (Raman, Brillouin, Rayleigh-wing, and thermal Rayleigh) of stimulated scattering were discovered in early 1960's, the stimulated scattering related studies have remained highly active over the past several decades because of both the fundamental research interest and the potential applications (Shen, Y. R., The Principles of Nonlinear Optics, New York: Wiley (1984); Boyd, R. W., Nonlinear Optics, Second Ed., Academic, San Diego (2002); He et al., Physics of Nonlinear Optics, World Scientific, Singapore (2000); and Kaiser et al., “Stimulated Rayleigh, Brillouin and Raman Spectroscopy,” in Arrecchi et al., eds., Laser Handbook, North Holland, Amsterdam (1972)). First, stimulated scattering is one of the most effective physical approaches to generate frequency-shifted coherent light emission. Second, stimulated scattering is one of the most effective technical approaches to generate optical phase-conjugate waves. In addition, the study of various stimulated scattering effects can provide a new knowledge and useful information about the interaction between nonlinear scattering media and intense coherent light radiation.
So far, for all known stimulated scattering effects, there is always a frequency shift between the stimulated scattering beam and the pump laser beam. For example, the frequency shift values are large (102˜103 cm−1) for most stimulated Raman scattering processes, which involve molecular vibrational transitions (Eckhardt et al., “Stimulated Raman Scattering from Organic Liquids,” Phys Rev Lett 9:455-457 (1962)). For backward stimulated Brillouin scattering in liquid and solid media, these values are quite small (10−1˜1 cm−1), corresponding to the frequencies of opto-electrostriction induced hypersonic waves (Chiao et al., “Stimulated Brillouin Scattering and Coherent Generation of Intense Hypersonic Waves,” Phys Rev Left 12:592-595 (1964)). In addition, the reported frequency shift values for stimulated Rayleigh-wing scattering were around several of cm−1 (Mash et al., “Stimulated Scattering of Light of the Rayleigh-line Wing,” JETP Lett 2:25-27 (1965)), whereas they are more than several hundreds of cm−1 for stimulated Kerr scattering (He et al., “Stimulated Kerr Scattering and Reorientation Work of Molecules in Liquid CS2,” Phys Rev A, 41:2687-2697 (1990)). These two types of stimulated emission have been observed only in Kerr liquids consisting of anisotropic molecules, and the frequency shift range is determined by the optical-field induced reorientation property of anisotropic liquid molecules.
There is another relatively unexplored effect named stimulated thermal Rayleigh scattering (Rank et al., “Stimulated Thermal Rayleigh Scattering,” Phys Rev Left 19:828-830 (1967); Cho et al., “Stimulated Thermal Rayleigh Scattering in Liquids,” Phys Rev 175:271-274 (1968)), which was observed by Rank and Cho et al. in linear absorbing media, and was explained by Herman and Gray using a theory of one-photon absorption enhanced thermal density fluctuation (Herman et al., “Theoretical Prediction of the Stimulated Thermal Rayleigh Scattering,” Phys Rev Lett 19:824-828 (1967); Gray et al., Phys Rev 181:374 (1969); and Batra et al., Phys Rev 185:396 (1969)). This theory could give an expression for the gain factor and predict an anti-Stokes shift that was about a half of the pump spectral line width, provided that the pump laser line width is much greater than the line width of spontaneous Rayleigh scattering for a given scattering medium. Although this precondition is fulfilled under most experimental conditions, there was a lack of experimental results to support the prediction of the anti-Stokes shift: some early reports partially supported this prediction (Rank et al., “Stimulated Thermal Rayleigh Scattering,” Phys Rev Left 19:828-830 (1967); Cho et al., “Stimulated Thermal Rayleigh Scattering in Liquids,” Phys Rev 175:271-274 (1968) and Pohl et al., “Experimental Observation of Stimulated Thermal Brillouin Scattering,” Phys Rev Lett 20:1141-1143 (1968)), whereas some others did not (Bespalov et al., “Stimulated Thermal Scattering of Short Light Pulses,” Phys Rev Lett 24:1274-1276 (1970) and Darée et al., “Competition Between Stimulated Brillouin and Rayleigh Scattering in Absorbing Media,” Phys Rev Lett 26:816-819 (1971)). According to the same theory (Herman et al., “Theoretical Prediction of the Stimulated Thermal Rayleigh Scattering,” Phys Rev Lett 19:824-828 (1967)), considerable linear (one-photon) absorption is needed to enhance the thermal fluctuation of a given scattering medium; however, on the other hand, a considerable linear attenuation would be highly harmful for the initial “seed” (spontaneous) scattering signal. One needs to remember that all other types of stimulated scattering always require that the chosen gain (scattering) medium should exhibit as little linear absorption as possible to lower the threshold of stimulated scattering. For this reason the so-called stimulated thermal Rayleigh scattering (STRS) might be quite difficult to observe, and that may be the reason why only a very small number of reports on this specific subject have appeared.
The possible two-photon absorption (2PA) contribution to the stimulated thermal scattering in pure organic solvents (such as benzene) was first reported by Boissel et al., “Stimulated Scattering Induced by Two-Photon Absorption,” Journal de Physique Lettres 39:319-322 (1978), although there was a lack of specific identification of the observed stimulated scattering. Later, the same possibility was also mentioned by Karpov et al. in another experimental study using other organic solvents (such as hexane) as the scattering media to generate backward STRS (Karpov et al., “Phase Conjugation of XeCl Excimer Laser Radiation by Excitation of Various Types of Stimulated Light Scattering,” Sov J Quantum Electro 21:1235-1238 (1991)). However, again there was a lack of specific identification of the stimulated scattering.
The present invention overcomes these deficiencies. This newly observed stimulated scattering shows no frequency shift and, therefore, is different from most other known stimulated scattering processes.